Protective device



Nov. 14, 1939. D. T. MA Y 2.179.826

PROTECTIVE DEVICE Filed April 7, 1958 -51 l Mia I??? INVENTOR D. 7. MA VA TTORNEV Patented Nov. 14, 1939 PROTECTIVE DEVICE David T. May, PortWashington, N. Y., assignor to Bell Telephone Laboratories,Incorporated, New York, N. Y., a corporation of New York ApplicationApril 7, 1938, Serial No. 200,623

7 Claims.

This invention relates to electrical systems in which current-carryingcircuits are opened and closed by switching devices, such as relaycontacts.

The objects of the invention are to protect the contacts utilized foropening and closing these circuits; to prevent excessive sparking; toeliminate surges from the circuits; and otherwise to effect improvementsin these systems.

In the process of producing successive electrical impulses for thepurpose of operating automatic switches in telephone systems, it isnecessary to repeatedly open and close circuits having inductiveelements therein, such as the windings of stepping magnets and relays.As it is the usual practice to open and close these circuits by means ofrelays, the contacts of these relays deteriorate, because of currentarcs formed at the moment of opening and closing, unless some circuitprovision is made to prevent it. One of the common safeguards is toconnect a condenser in parallel with the circuit controlling contacts ofthe relay. The condenser, when chosen with the proper capacitance, is avery emcient means for absorbing the surge that results from the openingof the contacts. However, when the contacts reclose, the entirecondenser charge is dissipated across the closing contacts, resulting ininjury and, in many cases, in the actual welding together of the contactmembers. To care for this condition, it is necessary to provide a seriesresistance to assist in dissipating the condenser discharge. Theinsertion of this resistance on the other hand lowers the efliciency ofthe condenser in absorbing the inductive energy of the surge producedwhen the contacts are opened. Thus, the design of contact protectioncircuits has in the past been a compromise between these two conditions.

According to the present invention the necessity of impairing theefficiency of the condenser in order to restrict its rate of dischargeto a safe value is obviated by connecting it across the contacts to beprotected in series with a variable-resistance element which responds totemperature changes and automatically assumes resistance values that arebest suited to both the open circuit condition and closed circuitcondition of the contacts. The normal resistance of thevariable-resistance unit is of such high magnitude that dischargecurrents from the series condenser are not permitted to flow insufficient intensity to injure the relay contacts. However, by eifectingan increase in its temperature, the resistance unit responds with animmediate drop in its resistance to a value such that current surges arepermitted to flow readily through the unit for the purpose of chargingthe condenser.

The protective arrangement is therefore equipped with a heating 0011 forthe variable-resistance unit, and these elementsthe condenser, theresistance unit, and the heating coil-are connected in the circuits insuch a way that the resistance unit is heated to place it in itslowresistance state when the current surges are to be absorbed by thecondenser and is cooled and correspondingly restored to itshigh-resistance condition when it is desirable to prevent the suddendischarge of the condenser through the relay contacts.

The foregoing and other features of the invention will be described indetail in the following specification, which should be considered inconjunction with the accompanying drawing.

In the drawing the invention is illustrated as applied to a selectorswitch in an automatic telephone system. The switch illustrated isof thestep-by-step type, such as those used in step-bystep central officesystems; For simplicity those parts of the selector not essential to anunderstanding of the present invention have been omitted in the drawing.For a full understanding of the details of these switches reference ismade to Automatic Telephony, second edition, by Smith and Campbell,pages 53 to 6'7. It will, of course, be understood that the invention isnot limited in its application to switches of this type; it may beapplied to other types of switches and in fact to other types ofsystems, including relay and key-controlled circuits, where protectionis needed for circuit contacts.

Referring particularly to the drawing, a telephone subscribers line I isshown appearing in the terminals of a line-finder switch F in thecentral oflice. The line finder F (which may be of any type known in theart) is associatedindividually with a selector switch S. The selector Sis equipped with an impulse relay 2, with setting and releasing magnets3 and 4, respectively, with controlling relays 5, 6 and I2, and withother operating and controlling circuit mechanism not disclosed in thedrawing. In addition to these devices the selector switch S is alsoprovided with a variable-resistance element 7 and its associated heatingcoil 8 for the purpose of protecting the contacts of the impulse relay2.

The variable-resistance element 1 may be made of any material, such asboron, silver sulphide or uranium oxide, having a high negativetemperature coefficient of resistance. If the elementis the impulserelay 2.

to be used for the protection of contacts that open and close at arelatively high rate, it is necessary to design the element and itsheating coil so that a correspondingly rapid change in the temperatureof the element can be secured. By applying sufficient energy to itsheating coil, the resistance element can be heated almostinstantaneously. And, by properly proportioning the'surfaces and volumeof the element, it can be made to lower its temperature at a rapid ratefollowing the removal of the heating current. While the resistanceelement disclosed herein may be of any suitable construction and of anydesired materials, a general understanding of elements of this type maybe had from British Patent 472,144, accepted September 17, 1937.

The resistance element F is included in series with a condenser 9 in acircuit which is, in effect, connected across the armature contact l0and its back contact ll. Ihe purpose of the element '2' in this circuitis to introduce a high resistance to the flow of discharge current fromthe condenser Q,- at times when this discharge current would damage thecontacts of relay 2, without offering any appreciable resistance to theflow of charging current in said circuit at times when it is desirableto allow current surges to flow into the condenser instead of flowingthrough the contacts of the relay. To this end the resistance element Iis equipped with a heating coil 8, which is placed in close proximity tosaid element. And, by causing current to flow in the coil 8 at theproper times, the resistance of the element i may be made to alternatebetween its high and low values.

A detaileddescription of the circuits will now be given. For thispurpose, assume that the subscriber of line i wishes to make a telephonecall. When he initiates the call, the line finder F operates in thewellknown manner to seize his line. Upon seizure of the calling line acircuit is closed from battery through the left winding of impulse relay2, inner back contact of relay i2, conductor .13, through the finderswitch F, over the loop of calling line 1, through the finder switch Fand conductor id, middle back contact of relay l2, through the rightwinding of relay 2 to ground. Relay 2 attracts its armature it, and acircuit is closed from ground through the outer back contact of relayl2, conductor ll, armature contact it! and its front contact it, windingof slow-release relay 5 to battery. Relay h attracts its armature andholds it in the attracted position during momentary interruptions of itscircuit by The subscriber now manipulates his dial it to open and closethe circuit of relay 2 any number of times in succession.

On the first opening or" the line i relay 2 releases its armature it,which engages the back contact Ii and closes the following circuit:Grounded conductor ll, armature it, back contact ll, front contact ofrelay 5, conductor is,

winding of relay 6, through the winding of'the vertical stepping magnet3 to battery. Magnet 3 operates in this circuit and advances the brushesof the selector S one step. The control relay 6 also operates. At thesame time a further circuit is closed from the grounded conductor llthrough armature It? and contact ll, front contact of relay 5, conductor58 through the heating coil 8 to battery. The coil it heats theresistance element 1, causing-it to assume its low-resistance condition.Nothing further occurs until the subscribers line i is reclosed at theend of the first impulse.

When the subscribers line does close at the end of the first impulse,the impulse relay 2 is reenergized and attracts its armature Ill. As thearmature it separates from its back contact II, the circuit of themagnet 3 and relay 6 is broken, and the inductance of these devicesproduces a high electromotive force which tends to continue the flow ofcurent inthe circuit. It is the surge produced by this relatively highinduced electromotive force that causes arcing across the contacts illand ii unless protection is provided. Since, however, the resistanceelement 1 is in its low-resistance state at the instant the surgeoccurs, the surge iiows over the following circuit and charges thecondenser 9 instead of producing an arc across the separating contactsill and II: Battery through the winding of magnet 3, winding of relay 6,condenser 9, resistor l in its lowresistance state to ground. Thus thecondenser 9 receives the inductive surge, and the contacts it and ii arepermitted to separate without detrimental sparking. Upon the opening ofcontacts l and ii the energizing circuit of coil 8 is opened, andresistance element begins to cool and to assume its high-resistancecondition. The time required for the element l to cool following thedeenergizati n of coil 3 is such that the ance of the charging circuitof condenser 9 is not increased appreciably until the contacts Ii) ithave separated enough to obviate any possibility of arcing. On the otherhand, the element ll cools with sumtcient rapidity, following thedeenergization of coil 3, to reach its full high value before thecircuit of relay 2 is opened for the next impulse.

When, after a moment, the line i opens again for the next impulse, relay*2 again releases its armature iii. As the armature iii approaches itsback contact ii, the condenser 92, being fully charged, would, unlessotherwise prevented, apply its full voltage across the gap of contactsit] and it. At this instant, however, the resistor l is in itshigh-resistance condition, limiting the how of discharge current fromcondenser 9 to a low value. No arcing occurs therefore as contacts it;and ii approach each other, and, when these contacts have finally closedthe condenser d discharges over the following circuit: Ground throughthe resistance element l, condenser 9, front contact of relay 5,contacts ii and iii to the grounded conductor i7. Since element 7 is atits high-resistance value at this instant, the intensity of thedischarge current is limited to a value which does not injure contactsiii and ll. As the contacts iii and ii close, the heating coil 8 isagain energized, and the element 1 begins to lower its resistance value.By the time element i has reached its low-resistance value condenser 9has partly discharged, and the remainder of the discharging action takesplace without injury to contacts it, and ii, particularly since thesecontacts are now fully closed and since the voltage across the terminalsof the condenser has been substantially lowered.

On the next closure of the line i, terminating the second impulse, relay2 attracts its armature Ell. As the armature iii recedes from contact llanother surge occurs in the coilsof magnet 3 and relay 6,,and this surgeagain flows into thecondenser 9 where it is stored instead of passingthrough the separating contacts l0 and II.

This action continues on each succeeding impulse, the surges beingstored in the condenser 9 andthe condenser 9 being discharged graduallywith each successive operation of the impulse relay.

After the selector switch S has been fully operated, the relay l2operates and extends the conductors l3 and I4 over conductors l9 andthrough the brushes of selector S to the next switch beyond.

When it is desired to release the connection, relay I2 is deenergized,and a circuit is completed from ground through the outermost contact ofsaid relay, conductor [1, armature I0 and contact ll, back contact ofrelay 5, off-normal contacts 2| through the winding of the releasemagnet 4 to battery. The magnet 4 energizes and restores the selectorswitch S to its normal position. The finder switch F is also restored inthe usual manner, and all other equipment utilized in the connection issimilarly returned to its normal condition.

What is claimed is:

1. In combination, a circuit having a source of current and an inductiveelement therein, relay contacts for closing said circuit to permitcurrent to flow from said source through said inductive element, aresistor having a pronounced temperature coefiicient of resistanceconnected in parallel to said relay contacts, a heating coil for saidresistor, and a circuit for said coil closed through said relaycontacts.

2. In combination, a circuit having a source of current and an inductiveelement therein, relay contacts for closing and opening said circuitsuccessively to cause impulses of current to flow through said inductiveelement, a resistor the resistance of which is variablebetweenrelatively high and low resistance values, a circuit for connecting saidresistor across said relay contacts, and circuit means made efiective bythe opening of said contacts for causing said resistor to assume itshigh-resistance value and made efiective by the closure of said contactsfor causing said resistor to assume its low-resistance value.

3. In combination, a circuit having a source of current and an inductiveelement therein, relay contacts for closing and opening said circuitsuccessively to cause impulses of current to flow through said inductiveelement, a resistor the resistance of which is variable betweenrelatively high and low resistance values, a circuit for connecting saidresistor across said relay contacts, and means controlled by said relaycontacts for causing said resistor .to assume its high-resistance valueprior to each closure of said contacts and to assume its low-resistancevalue prior to each opening of said contacts.

4. In combination, a circuit having current and an inductive elementtherein, relay contacts for closing and opening said circuitsuccessively to cause impulses of current to flow a source of throughsaid inductive element, a resistor the resistance of which varies withits temperature between relatively high and low values, a circuit forconnecting said resistor in parallel with relay contacts, and heatingmeans controlled by said relay contacts for causing said resistor topresent a high resistance to the flow of current at the closure of saidcontacts and to present a low resistance to the flow of current at theopening of said contacts.

5. In combination, an impulse circuit having an inductive element and asource of current therein, contacts in said circuit, means for openingand closing said contacts to produce impulses of current in saidcircuit, a condenser for storing the currents induced by said inductiveelement upon the opening of said impulse circuit, a charging circuit anda discharge circuit for said condenser, a resistor the resistance ofwhich is variable between upper and lower values, said resistor beingincluded in both said charging and discharge circuits, and means forcausing said resistor to present a relatively low resistance to the flowof current in said charging circuit and a relatively high resistance tothe flow of current in said discharge circuit.

6. In combination, an impulse circuit having an inductive element and asource of current therein, contacts in said circuit, means for openingand closing said contacts to produce impulses of current in saidcircuit, a condenser for storing the currents induced by said inductiveelement upon each opening of said contacts and for discharging saidcurrents oneach closure of said contacts, a charging circuit for saidcondenser excluding said contacts, a discharge circuit for saidcondenser including said contacts, a resistor the resistance of which isvariable between high and low values, said resistor connected in serieswith said condenser in both the charging and discharge circuits, andmeans controlled by said contacts for causing said resistor to present alow resistance to the flow of current in said charging circuit when saidcontacts open and to present a high resistance to the flow of current insaid discharge circuit when said contacts close.

'7. In combination, a circuit having a source oi current and animpedance element therein, relay contacts for closing and opening saidcircuit sue cessively to cause impulses of current to flow through saidimpedance element, a resistor having a negative temperature coefficientof resistance, a circuit for connecting said resistor across said relaycontacts, and means for causing said resistor to assume itshigh-resistance value preparatory to the closure of said contacts andits low-resistance value preparatory to the opening of said contacts.

' DAVID T. MAY.

